The present invention relates to a method of bonding a wire to a pad of an electronic circuit and, more particularly, to a method of bonding a number of aluminum wires to a very small area such as that in a hybrid module.
An electronic circuit module such as, for example, a hybrid module is constructed such that semiconductor pellets are mounted upon a multi-layer ceramic module. In the electronic module, it is often necessary to modify the wiring due to changes or errors in the design of the module. When modifying the module, it is impossible to renew the wiring of the module per se each time such modification is made and, in such situations the bonding technique employing an engineering change pad (EC pad) which is provided on the module and onto which discrete wires are attached, is adopted. A procedure of this type is described in, for example, "Precise Numerical Control for the Thermal Conduction Module" by M. A. Sanborn of IBM J. Res. Den. (Vol. 27. No. 1). Wires employed with the engineering pads must have a small diameter with a higher strength and conductivity than usually employed, and have an insulation coated thereto.
Conventionally, a copper wire is used as the wire satisfying the necessary requirements, with the copper wire being plated with Au for preventing a deterioration of bonding due to oxidization and also has its surface thereof coated with an insulating film of a resin such as, for example, TEFLON, polyurethane, etc.
A general method of bonding proposes removing the insulation film by ultrasonic waves and thereafter to effect a bonding by thermocompression-bonding. However, a disadvantage of this general method resides in the fact that the coated wire becomes thick thereby enlarging the bonding area. Further, the proposed method entails a two step operation; namely, removing the coated film and then performing the thermocompression bonding, thereby increasing the overall processing time. Moreover, the removal of the coated film as a whole is difficult and, consequently reliability of the bonding is low. These disadvantages are not limited to the wire bonding for engineering change, but can generally be involved whenever wire bonding is effected with respect to very small areas.
The object of the present invention is to provide a method capable of bonding wires with high reliability in very small areas.
The smaller the bonding area for wires, the smaller the wire diameter is necessary and, to this end, the coating on the wire is preferably small in thickness. In the bonding method according to the present invention the working efficiency increases when the coating removal and the bonding can be carried out at the same time. Further, it is preferable from the viewpoint of reliability that no coating residue is left on the bonding section. To meet these demands, the present invention is arranged such that, while a load is being applied onto an anodized aluminum wire by pressing the same against a pad by means of a wedge, ultrasonic vibrations are imparted to this wedge, thereby effecting the bonding of the wire.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for the purposes of illustraton only, one embodiment in accordance with the present invention.